In the coming year we hope to study the physiological significance of the bacteriophage T4 induced modification of valyl-tRNA synthesis. To date the modification proceeds in the following way. A phage gene is transcribed as an immediate-early function. The product of the gene is a peptide tau. The peptide is added to valyl-tRNA synthetase of E. coli. As a result the enzyme, now called modified, interacts more strongly with many species of tRNA but continues to aminoacylate only the valine species. Phage mutants, deficient in modification, develop normally under usual laboratory conditions. Our data, however, indicate that these mutants are leaky in that they partially modify the host enzyme. Future experiment will, therefore, utilize a strain of E. coli that has a damaged valyl-tRNA synthetase. At non-permissive conditions (high temperature) this strain does not grow certain vs gene mutants. We hope by looking at such infected systems both in vivo and in vitro to assess the value of modification to the programming of synthesis of other viral gene products. In addition we are developing a radioimmune assay which will utilize the reagent methyl-p-hydroxy benzimidate to introduce I125 probes into the tau molecule at free amino groups. A radioimmune assay would allow us to study the intracellular localization of tau as well as its regulation in phage-infected bacteria.